The market for home networking is developing at a phenomenal rate. Service providers from cable television, telephony and digital subscriber line markets are vying to deliver bundled services such as basic telephone service, Internet access and entertainment directly to the consumer. Collectively these services require a high-bandwidth network that can deliver 30 Mbits/s or even high rates. The Institute of Electrical and Electronic Engineers (IEEE) 802.11a standard describes a cost-effective, robust, high-performance local-area network (LAN) technology for distributing this multimedia information within the home. Networks that will operate in accordance with standard 802.11a will use the 5-GHz UNII (unlicensed National Information Infrastructure) band and may achieve data rates as high as 54 Mbits/s, a significant improvement over other standards-based wireless technology. The 802.11a standard has some unique and distinct advantages over other wireless standards in that it uses orthogonal frequency-division multiplexing (OFDM) as opposed to spread spectrum, and it operates in the clean band of frequencies at 5 GHz.
OFDM is a technology that resolves many of the problems associated with the indoor wireless environment. Indoor environments such as homes and offices are difficult because the radio system has to deal with a phenomenon called “multipath.” Multipath is the effect of multiple received radio signals coming from reflections off walls, ceilings, floors, furniture, people and other objects. In addition, the radio has to deal with another frequency phenomenon called “fading,” where blockage of the signal occurs due to objects or the position of a communications device (e.g., telephone, TV) relative to the transceiver that gives the device access to the cables or wires of the cable TV, telephone or internet provider.
OFDM has been designed to deal with these phenomena and at the same time utilize spectrum more efficiently than spread spectrum to significantly increase performance. Ratified in 1999, the IEEE 802.11a standard significantly increases the performance (54 Mbits/s vs. 11 Mbits/s) of indoor wireless networks.
The ability of OFDM to deal with multipath and fading is due to the nature of OFDM modulation. OFDM modulation is essentially the simultaneous transmission of a large number of narrow band carriers sometimes called subcarriers, each modulated with a low data rate, but the sum total yielding a very high data rate. FIG. 1a illustrates the frequency spectrum of multiple modulated subcarriers in an OFDM system. To obtain high spectral efficiency the frequency response of the subcarriers are overlapping and orthogonal, hence the name OFDM. Each narrowband subcarrier can be modulated using various modulation formats such as binary phase shift keying (BPSK), quatenary phase shift keying (QPSK) and quadrature amplitude modulation QAM (or the differential equivalents). The 802.11a specifies that each 20 MHz channel has 52 subcarriers covering 16.5 MHz of the 20 MHz , leaving 3.5 MHz to be used for preventing interference between channels.
Since the modulation rate on each subcarrier is very low, each subcarrier experiences flat fading in multipath environment and is relatively simple to equalize, where coherent modulation is used. The spectrums of the modulated subcarriers in an OFDM system are not separated but overlap. The reason why the information transmitted over the carriers can still be separated is the so-called orthogonality relation giving the method its name. The orthogonality relation of the subcarriers requires the subcarriers to be spaced in such a way that at the frequency where the received signal is evaluated all other signals are zero. In order for this orthogonality to be preserved it helps for the following to be true:                1. Synchronization of the receiver and transmitter. This means they should assume the same modulation frequency and the same time-scale for transmission (which usually is not the case).        2. The analog components, part of transmitter and receiver, are of high quality.        3. The multipath channel needs to accounted for by placing guard intervals which do not carry information between data symbols. This means that some parts of the signal cannot be used to transmit information.        
IEEE 802.11a standard defines the structure of a packet that is used for information transmission between two transceivers. A receiver derives timing information, data, and other information from the packet. For example, the first 10 symbols (t1 to t10) in the packet are referred to as the shorts; repeated random sequences that a receiver uses for detecting symbol timing and coarse carrier frequency offset. A guard interval (GI1) follows the shorts and acts as a rough inter-symbol boundary for absorbing the effect of multipath. The guard interval is made long enough such that if short symbol t10 undergoes multipath, symbol t10 will partially “smear” into GI1 without affecting the first long symbol (T1) that follows the shorts. A receiver may receive noise that may cause the receiver to commence processing of the noise as though it were the start of the short symbols. If the receiver fails to detect the false detection relatively quickly, there is the possibility that the receiver will continue to process the noise and fail to process a legitimate packet. The present invention provides a mechanism for detecting the false detection of the start of a packet, thereby, allowing the receiver to return relatively quickly to waiting for a legitimate packet.